This invention relates to a gas migration control system and, more particularly, to a system for controlling gas migration in a sanitary landfill.
It is a common practice in developing a land area to construct a landfill, the landfill generally being formed from garbage, refuse, and the like. In such a sanitary landfill, decomposition gases are generated, which typically comprise largely carbon dioxide and methane, although other odorous gases such as hydrogen sulfide may also be generated. Methane is a colorless, odorless gas that is explosive in concentrations of 5 - 15%, and so provision must be made for dissipating these gases, inasmuch as they are generated by a sanitary landfill for many years. These gases are also generated by marsh deposits such as organic silt and peat, upon which the landfill is often placed.
If the decomposition gases are not properly removed from the landfill, methane gas may accumulate in explosive concentrations in sewer lines and beneath or within buildings located not only in or above the landfill, but also adjacent thereto, inasmuch as methane will migrate laterally from the landfill if the material adjacent thereto is more permeable than the material covering the landfill.
Among the efforts that have been made to control gas migration in a landfill has been the construction of one or more wells or shafts in or adjacent to a landfill, the decomposition gases being withdrawn by one or more exhaust blowers. Activated carbon filters have been employed to remove odors from the gases discharged by the blowers, but such filters do not absorb methane. In addition, incinerators have been installed to burn the methane and the odorous gases, natural gas being supplied as a pilot if the concentration of methane is too low to support combustion.
Gravel-filled trenches have also been constructed, the trenches generally extending below the landfill in an attempt to intercept all lateral gas flow. Where a relatively impermeable top cover, such as clay or pavement, overlies the landfill, a system of relatively shallow gravel-filled trenches has been constructed at the top of the landfill, the gases being discharged through vertical pipes extending through the top cover and connected to perforated, lateral pipes in the trenches. In some instances the gases discharged from the vertical risers have been burned off.
A technique used to contain or block the decomposition gases has been to construct an impermeable barrier in the form of a liner or a wall adjacent to the landfill. Generally, a layer of clay 1 - 4 feet thick, for example, or a synthetic membrane has been used for such barrier.
If buildings are to be constructed over a landfill, the gases generated under the building slab could collect in sufficient quantities to cause a fire and/or an explosion. Furthermore, these gases could infiltrate into the building so as to create a hazardous condition there. In order to avoid such danger, subfloor ventilation systems have been constructed in order to dilute the decomposition gases with fresh air and to exhaust these gases before dangerous concentrations can accumulate. Such exhaust systems include a plurality of fresh air trenches vented to the atmosphere, connected to which are a plurality of perforated pipes imbedded in gravel-filled trenches extending beneath the building slab, the gravel-filled trenches being connected with exhaust fans by means of a plurality of imperforate exhaust ducts.